Battery Sizing and Ratings

To determine the size of a battery required to run a series of electrical appliances in (say) a 4WD off-road vehicle, with fridge, lighting etc., use the following guidelines:

  • Determine the current draw in amps of the various equipment: e.g. Fridge: 3A, lights: 10A, heater: 25A, TV: 2A (Note: Divide Watts by Volts to determine Amps)
  • Estimate the hours each will run per day: e.g. Fridge: 8 hours, Lights: 4 hours, heater: 1 hour, TV: 8 hours.
  • Multiply the hours by amps to determine the daily amp-hours used by each item: Fridge: 3X8=24, Lights: 4X10=40, heater: 25X1=25, TV: 2X8=16.
  • Total the amp hours: 24+40+25+16=105AH

As you should not discharge a battery below 50% of its capacity on a regular basis, double this total. (105x2=210AH) This is minimum battery capacity required to provide reasonable service, assuming the battery can be recharged to full capacity during the next day, which may take 8 hours at 15-18amps charging. For the above example, if the applications are all 12 volt, two 6 Volt 220 Ampere Hour golf cart batteries connected in series will do the task.

Deep Cycle Battery Ratings

  • 20 Hour Rate
    This is the most universal rating, and is expressed in Ampere Hour (AH). It indicates the capacity of the battery, using a fixed current draw to reach a rating of 1.75 Volts per cell (10.5V for a 12 V battery), at which level the battery is around 90% discharged. The current to sustain a 20-hour rate discharge is calculated by dividing the 20Hr capacity by 20. e.g. 220AH (golf buggy battery) can sustain a discharge current of 11 amps for 20 hours (220/20 = 11)

  • 6 hour, 5 hour, 60 minute ratings
    These have the same test parameters as the 20-hour rating, just shorter discharge times. As the power is discharged more rapidly, less current is available to the voltage cut off point. These ratings are used in the materials handling industry, where batteries are used in electric vehicles or scissor lifts etc.
    For quick calculation, available power can be estimated by multiplying the 20-hour rate by a factor:
    6Hr capacity = 20 hour rate X0.84
    5Hr capacity = 20 hour rate X0.82
    1 hour capacity = 20Hr rate by 0.59
    30 min capacity = 20hour X0.5
    These fast discharges don’t remove all power that is available, however this is all that can be provided by the battery to the voltage cut off point at that discharge rate.

  • 75A discharge
    Commonly used in the golf industry, and expressed as minutes. This provides an idea of how long the golf cart can operate, as its current draw is around 75 amps when moving.
    e.g. GC2200 has a 75A rate of 108 minutes.
    (Note electric golf carts are not drawing current all the time during 18 holes, they only use current when moving, not when stationary. During a 3-4 hour game, the carts may only be in use for 20-30% of the time).

Temperature affect on capacity
The ideal operating temperature for deep cycle batteries 25 degrees centigrade. Extreme cold or heat affects capacity and charging characteristics. To estimate the affect on cold performance for golf buggies, or other battery use, use the following estimates of reduced capacity at various low temperatures:
12 C = 0.85
0 C = 0.75
-7 C = 075
-18 C = 0.55

e.g. A golf cart battery of 220AH rating, will only provide 165 (220X0.75) amp hours at a temperature of 0 degree Centigrade........... You may need to walk the last few holes, although the battery will still provide 108minX0.75 = 81mins of drive time. This should be OK, and besides at 0 Centigrade, the walk will keep the ice off your toes.